13 Turbocharging

Page 1

Turbocharging is done by two different techniques: Constant-pressure turbocharging, where the exhaust manifolds from all cylinders are connected to a common large chamber, where pressure is nearly constant. The exhaust ports of all cylinders are connected by short pipes to a single manifold whose volume is sufficiently large to damp the pressure pulses coming from each cylinder. If these pulses are not fully damped they could destruct the scavenging of another cylinder. In this system there is a degradation of energy: the work that can be extract from the turbine is less than the one available in the cylinder. Advantages: 1) Conditions at the turbine inlet are nearly steady with time, resulting in a simple design and high efficiency 2) Structure of exhaust manifold is compact and cheap 3) high supercharging pressure can be reached Disadvantages: 1) poor efficiency in energy transmission from cylinder to turbine 2) difficult matching engine-turbocharger over wide speed and load range 3) poor turbocharger response when there is a sudden increase of load/speed Therefore constant pressure turbocharger is not appropriate for automotive engines, while it is widely used in stationary Diesel engines. Pulse turbocharging, where connection between each cylinder of the engine and the turbine is made by narrow pipes, so that most of the kinetic energy of the gases leaving the exhaust ports is maintained and not destroyed in a large volume. The kinetic energy of the gases is partially transmitted to the turbine in the form of pressure pulses. When the pressure wave travels along the pipe, it carries a large portion of pressure energy and a small amount of kinetic energy. The turbine therefore receives a good amount of energy at any condition of engine operation is available.…